A moving-object sensor of the kind disclosed in Blanyer U.S. Pat. No. 3,721,859 generates a distinctive group of signals indicative of the movement of a railway wheel or other sensed object into and out of a sensing space or region immediately adjacent to the sensor. These signals are not simple pulses. One of the signals exhibits a substantial excursion of one polarity indicative of intrusion of an object into the sensing space at the left-hand end of the sensor and an excursion of opposite polarity representative of intrusion of an object into the sensing space at the right-hand end of the sensor. These two excursions can be treated separately as individual left-hand and right-hand recognition signals. A third signal in the group produced by the sensor is a center recognition signal representative of intrusion of an object into the center portion of the sensing region adjacent the sensor. Ideally, each of these output signals from the sensor is zero in the absence of an object to be sensed, and each reaches and dwells at a predetermined maximum amplitude during passage of an object through the sensor. In actual service, the at-rest signal values depart from zero to a greater or lesser extent as the result of various non-ideal operational conditions. Furthermore, during the time that an object actually passes through the sensor, any or all of the recognition signals may reach the normal maximum amplitude only very briefly or may even fail to reach that level. Thus, the waveforms of the recognition signals may vary to a great extent.
Waveforms much simpler than those provided by the wheel sensor referred to above would be quite adequate for simple utilization apparatus requiring only information about passage of an object, with no necessity for determining the direction or speed of the object. In many applications, however, it is essential to detect direction as the object moves past the sensor; this is particularly true in railway and other vehicular applications. Effective detection of direction is rather difficult where a broad speed range must be accommodated, particularly if that range extends to zero speed. A complicating factor, again particularly prevalent in vehicular systems, is the possibility, by no means negligible, that the object may stop within the sensing region adjacent a sensor and may then proceed away from the sensor in either its original direction or back in the direction from which it came. A further basic difficulty is that the recognition signals developed by the sensor are often generated in response to movement of different objects (e.g., the several wheels of a railway truck) past the sensor, rather than from the same object, regardless of whether traffic is all proceeding in one direction or is reversing from time to time. Furthermore, time and duration information, relative to the sensor signals, cannot be employed in any simple or trustworthy fashion to unravel this complexity.
A primary requirement in utilization of the output signals from a moving-object sensor or other like sensing device is, of course, discrimination between a true object presence signal and the various drifting and fluctuating signals that may be developed at the sensor outputs even when there is no object present. Thus, a preferred first step in any utilization circuit is the establishment of an operational threshold for each of the recognition signals, so that signal amplitudes below the threshold level are treated as if they had not occurred. In many cases it is sufficient to provide a simple yes-no mode of operation for signals that exceed the threshold amplitude, thus converting the sensor output signals to simple on-off signals.
If signal amplitudes below the threshold are ignored, there is always a "zero signal" interval of finite duration between the left-end and right-end recognition signals developed by the moving-object sensors of U.S. Pat. No. 3,721,859 and similar sensors. The center recognition signal must occur during that zero-signal interval. In many applications, the useful portion of the center recognition signal overlaps each of the left-end and right-end recognition signals in time; two such signal overlaps occur each time an object moves completely through the sensor region. To obtain these overlaps on a consistent basis, however, the operational threshold may have to be made relatively low with respect to the smallest signal peaks developed by the sensor.
The aforementioned overlap between signals has been effectively used to generate object-passage signals that include directional information. A flaw common to all such arrangements is the dependency on relative timing of the center recognition signal with respect to the two end recognition signals. If the operational threshold is raised or the peak amplitude of the output signals from the sensor is lowered, the duration of the overlaps between the center recognition signal and the end recognition signals is reduced; indeed, the probability of the existence of any overlap is also reduced. Thus, a low operational threshold for the recognition signals contributes substantially to security as regards the integrity of directional information. On the other hand, a high operational threshold contributes to security as regards effective discrimination between true object-produced signals and transients or other extraneous signals. Accordingly, those arrangements in which directional data is derived from signal overlap may require a compromise, with respect to the operating threshold, that is rather unsatisfactory and may lead to occasional erroneous interpretation of the output signals by the utilization equipment to which they are supplied. This can be particularly dangerous in vehicular applications in which the output from the moving-object sensor is relied upon to indicate the state of occupancy of a railway track, a section of roadway, a multiple-input conveyor, or the like.